Error B7 257

Safety precautions:

- Do not work on gas or mains electrical parts unless you are a qualified engineer. Electricity and gas are dangerous.

- Isolate electrical supply at the fused spur or consumer unit before removing any covers or touching internal components. If unsure, stop and call an engineer.

- If you smell gas, evacuate the property and call the gas emergency number immediately.

- Take photos of connections and the error display before attempting anything so the engineer has full information.

Initial homeowner checks (safe, outside the boiler casing):

1. Note the exact fault code and cause code displayed (B7 257). Write down the boiler model and serial number and the date/time the fault occurred.

2. Check the boiler has power: confirm the fused spur or switch to the boiler is on and that other appliances on the same circuit are working.

3. Check basic system conditions: water pressure at the boiler is about 1–1.5 bar when cold, and there are no obvious leaks or water around the boiler base or on wiring.

4. Perform one controlled reset: using the boiler’s reset button or turning the boiler off for 30 seconds and back on. If the error clears once and does not return, monitor the boiler. Do not repeatedly reset the boiler if the fault returns immediately.

If the fault persists after a single reset or returns:

5. Visually inspect the outside of the boiler for signs of water ingress, corrosion, burn marks or damaged wiring into the casing. Do not open the casing unless you are competent and have isolated the power.

6. Check for obvious loose external connections: external controls, room thermostat wiring and any visible plug-in connections used by external controllers. Ensure external controls are powered and calling for heat (if relevant).

Safe, limited internal checks only if you are competent and have isolated electrical supply:

7. Isolate electrical supply at the fused spur and wait a minute. Remove the casing only if you are confident and able to re-secure it correctly. Look for damp, water staining, or obvious scorched components on the control board and the HCM/code plug area.

8. Check connector seating: gently reseat plug-in connectors to the HCM and control board (do not force). Take photos so connectors can be reassembled the same way. Do not cut or strip wiring.

9. If you find water, corrosion, or burn damage, do not attempt to repair the PCB; this needs an engineer and likely replacement parts.

When to call a professional (required in most cases):

10. Call a Gas Safe registered engineer or Worcester accredited service if the B7 257 persists after the basic checks, if you found internal damage or water ingress, or if you are not comfortable performing the safe checks above. Explain the exact fault code (B7 257), the cause code if shown, and any symptoms (no heat, no hot water, boiler locked out).

11. The engineer will perform detailed electronic diagnostics, test the HCM/code plug and PCB, check wiring harnesses, and replace faulty modules if necessary. PCB or HCM replacement should only be performed by a qualified engineer because of gas and electrical safety, software/code plug matching, and commissioning requirements.

Other practical notes for the engineer call:

- Have model, serial number and the exact error and cause codes ready.

- If the boiler is under warranty or a service contract, contact Worcester or your installer first.

- Avoid repeated resets; they can obscure fault history and may cause additional wear.

Summary: B7 257 is an internal control/HCM/PCB type fault. Perform safe external checks and one reset, but expect a Gas Safe engineer to diagnose and replace control electronics if the fault persists or if there is evidence of damage.